JP3485818B2 - Method for stabilizing the weight of green compact in powder molding process of sintered parts - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a powder forming process in the process of mass producing sintered parts,
More specifically, the present invention relates to a method for eliminating the weight fluctuation of the green compact in the initial stage after the start of continuous molding and mass-producing the green compact having a stable weight from the beginning.

[0002]

2. Description of the Related Art A powder molding press used in the fields of powder metallurgy, ceramics, etc. usually has a die 10 mounted on a die plate 11 as shown in FIG. The die cavity formed by the penetrating core rod 40 is filled with the raw material powder from the powder feeding device, and the powder in the die is compression-molded between the upper and lower punches. The powder feeding device is elevated to feed the raw material powder. The hopper 60 for storing and the box-shaped feeder 70 without a bottom are connected by a flexible hose 61. The feeder advances from its standby position toward the die (to the left in the figure), pours the raw material powder onto the die 10, and then flattens the upper surface of the powder deposited in the die at the lower edge of the feeder. That is, while scraping off, it retreats to the standby position and completes one filling cycle. Reference numeral 71 in the figure denotes a drive rod that is connected to a drive device (not shown) such as a hydraulic cylinder to move the feeder forward and backward.

FIG. 2 exemplifies the structure of a die set including the above-mentioned die, and the lower punch 20 is a ground plate 2
It is fixed to the base of the press through 1. While die 1
0 is fixed to the die plate 11, the core rod 40 is fixed to the column plate 13 attached to the lower ram 50 of the press, and the die plate and the column plate are connected by the column 12 penetrating the ground plate. Therefore, the die 10 and the core rod 40 move up and down at the same time as the lower ram 50 moves up and down. The left side of the figure shows the die 1 so that the cavity depth (filling depth) will be the specified depth.
The state where 0 is positioned and the powder is filled is shown, and the right side of the figure shows the state where the upper punch 30 is lowered and the powder in the die is compression-molded with the lower punch 20. The top dead center of the stroke in which the die moves up and down and the bottom dead center of the stroke of the upper punch can be adjusted arbitrarily.
Since this is a known matter, its explanation is omitted here.

The size of the green compact is determined in consideration of spring back generated when the green compact is extruded from the mold, dimensional change due to sintering of the green compact, necessity of sizing after sintering, and the like.
It is designed by calculating back from the specifications and dimensions of the sintered product. When the height of the green compact is determined in this way, the filling depth of the powder is calculated by multiplying the height of the green compact by the compression ratio of the powder. Here, compression ratio = density of green compact (green density) / apparent density of green powder
Is. However, mass production molding is not performed as it is with this calculated value. In the actual molding process, the filling depth is set to this value and the press and mold are set and trial punching is performed.
Mass production is started after making necessary adjustments according to the measured values such as the weight and height of the obtained green compact.

[0005]

Since such adjustments have been made, the specifications such as the weight of the green compact should originally have given values from the beginning of mass production molding, but in reality There is a problem in not becoming. As an example, iron powder is mixed with 1.5% copper powder and 0.8% graphite powder, and 1% zinc stearate as a powder lubricant (all are% by weight) to obtain a raw material powder. 5. The inner diameter is 30 mm, the outer diameter is 45 mm, and the height is 20 mm.
A green compact of 7 g / cm ³ was continuously molded at a molding speed of 10 pieces per minute, and the weight of each of the resulting green compacts was measured. The results are shown in the graph of FIG. 3 as a conventional example. The rating is 1 for molding
A mechanical powder press of 00 tons is used, and the weight of the green compact is measured by a direct balance.

In this graph, the weight data of the green compact is divided into 10 zones in the order of molding, the average value of the data in each zone is calculated, and the weight of the first green compact is measured on the scale 1 on the horizontal axis. Here, the average value of the first to tenth compacts is graduated 1
The average value of the 0, 11th to 20th green compacts is shown on the scale 20. Although the number of moldings is scaled on the horizontal axis of the graph, it also means elapsed time from the relationship with the above-mentioned molding speed, and 100 moldings corresponds to 10 minutes in time.

As shown in this graph, the weight of the green compact is a predetermined value (118.38) as adjusted immediately after the start of molding.
g), but after 4 minutes 118.64 g, after 8 minutes 119.12 g ... It gradually increases with the passage of time. And this increase was settled in about 100 pieces in the number of moldings and about 10 minutes in time, and after that, it remained in a substantially stable state (119.45 g on average). Regarding the weight variation, in the weight variation section (horizontal axis scale 10 to 100), the average value of the variation range (difference between the maximum value and the minimum value of 10 data) in each area is 0.25, weight. Stable section (horizontal axis scale 120 to 3
The average value of the range in 10) was 0.26.

Therefore, in this case, the variation itself remains constant from beginning to end, and the green compact is mass-produced in a state of being biased toward the average 1.07 g, that is, about 0.9% heavier than the normal weight. The tendency is that even if the molding speed or the type of raw material powder is changed, the same pattern is obtained although the individual values are different.

Such a variation in weight is usually caused for several minutes to several tens of minutes after the start of molding. There is a concern that the body will be mass-produced. By the way, even if the weight changes, it is possible to make the height of the green compact uniform, but in that case, the green density changes. When the green compact density changes, the sintered density changes, which affects the strength and other mechanical properties of the product. Therefore, it is necessary to repeat the adjustment of the filling amount (that is, the filling depth) according to the change of the weight throughout the change of the weight, which is one of the causes of impairing the efficiency and the cost.

[0010]

The variation of the weight of the green compact is the variation of the amount of the raw material powder filled in the die cavity. This may be due to equipment-side factors such as variations in die cavity filling depth due to defective mold operation, and powder-side factors such as fluidity and apparent density variations of the powder. Since it was constant, it is presumed that the powder characteristics of the raw material powder fluctuated for some reason between the initial stage and the subsequent stage of molding. Therefore, as a result of measuring the temperature of each part of the green compact and the mold through the molding process in order to elucidate the cause, between the temperature of the die mainly including the die and the weight of the green compact, as described below. A kind of correlation was found.

FIG. 4 is a graph showing the results of sampling the temperature of the green compact immediately after being extruded from the die by the surface thermometer, every 10th, 10th, 20th ... The temperature of the powder rises sharply up to the 40th point and then moderately thereafter, and remains almost constant after the 100th point. The data at the main points in the figure are the first ... 43.3
° C, 10th ... 49.6 ° C, 40th ... 55.2 ° C, 1
00th ... 58.8 ° C, temperature is stable 12
The average temperature in the section from No. 0 to No. 310 is 59.3 ° C.

Further, the temperature of the mold before and after molding was measured with a surface thermometer.
The temperature was 27.3 ° C., upper punch ... 26.7 ° C., and the temperature immediately after the completion of molding was die ... 42.8 ° C., upper punch ... 42.0 ° C. Therefore, the temperature of the mold is 15.
The upper punch shows an increase of 15.3 ° C. at 5 ° C., which means that the green compact has an average increase of 16 ° C. in the equilibrium state. Such temperature changes of the green compact and the mold are considered as follows. That is, the green compact heats up due to friction between the powders when the powders are compression-molded and frictional resistance with the die when the compression-molded green compacts are extruded from the mold, and naturally becomes hotter than the raw powders. . On the other hand, the die also generates heat due to friction with the powder during operation, particularly frictional resistance when the green compact is pushed out of the die. The temperature of the mold gradually rises due to this frictional heat accumulated as the molding is repeated, but eventually, the equilibrium state is reached in balance with the heat radiation to the surroundings.

When the temperature of the die rises in this manner, the friction heat of the die is added to its own frictional heat, so that the temperature of the die rises as the temperature of the die at the time of molding rises. However, it is considered that the temperature of the green compact also becomes constant when the mold temperature reaches an equilibrium state. By the way, the temperature difference between the green compact and the die is 1 at the start of molding according to the above data.
At 6 ° C, the average at the end is 16.5 ° C, so it can be seen that the temperature difference between the two is almost constant regardless of the elapsed time. This means that the graph of FIG. 4 directly shows the time-dependent change in the temperature of the green compact, but indirectly implies the temperature of the mold, that is, instead of measuring the mold temperature during operation. In addition, it means that it does not matter if the graph obtained by moving this curve downward by about 16 ° C. on the ordinate scale is regarded as showing the time-dependent change of the mold temperature.

By comparing the graphs of FIGS. 4 and 3, it can be seen that there is a close relationship between the change in the weight of the green compact and the change in the temperature with the progress of continuous molding. That is, both the weight and temperature of the green compact are remarkably increased from the start of continuous molding to a certain point, and the equilibrium state is maintained thereafter, and the time to reach the equilibrium state is almost the same. The sex is outstanding. And this fact is that the weight of the green compact also increases while the mold temperature rises, and the pressure increases when the mold temperature reaches an equilibrium state, as described above regarding the relationship between the temperature of the green compact and the mold temperature. The weight of the powder also means entering equilibrium.

The reason why the weight of the green compact and the mold temperature correlate in this way is considered as follows. That is, since the raw material powder filled in the die cavity receives heat from the surrounding molds both during the inflow and after the deposition, the higher the mold temperature, the higher the temperature of the raw material powder, and the powder lubricant added. Promotes softening of. Then, as a total effect such as change in powder fluidity and apparent density, and settling of accumulated powder, the higher the temperature of the mold, the more the amount of raw material powder filled, and hence the weight of the green compact. Therefore, if the temperature of the mold is raised to the expected equilibrium temperature in advance and the filling amount of the raw material powder is adjusted to a predetermined value in that state and molding is started, the temperature has already risen to the mold. There is no room for temperature, so the weight of the green compact should not change, and the initial set value should be maintained within the allowable error range.

The present invention has been made on the basis of such knowledge, that is, the present invention obtains the equilibrium temperature reached by the temperature of the mold, which heats up due to repeated molding, by empirical rules or preliminary tests. The main idea is to preheat the mold to its equilibrium temperature, adjust the filling amount of the raw material powder to a predetermined value in that state, and start molding. In this case, the target of preheating is the die, core rod, lower punch, etc. involved in the formation of the die cavity, and preheating is not essential for the upper punch that does not contribute to the filling amount of the raw material powder. There may be no core rod depending on the shape of the green compact.

The equilibrium temperature of the mold during operation depends on the type of powder, the shape and size of the green compact, the molding pressure and speed, the material of the mold,
Although it is affected by various conditions such as surface roughness, in most cases, it is within the range of 10 to 30 ° C. added to the mold temperature before the start of molding. The preheating means is optional, but in terms of cost and labor, it is preferable to blow hot air by electric heating. The temperature of the feeder also rises slightly due to sliding friction with the die plate and heat transfer from the die, but it is not as high as that of the mold. If the temperature of the feeder becomes too high, it may affect the powder properties, so it is better to be conservative when preheating the feeder after the mold.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION In the following examples, the raw material powder, the mold and the powder molding press are the same as those in the conventional example of FIG. 3 for the sake of comparison. The die temperature before molding is 27 ℃
Therefore, considering the rise temperature of the die in the conventional example, +
It was decided to heat at 16 ° C, and hot air was blown to preheat the dies such as the die and core rod to 43 ° C. Then, in this state, the powder filling amount is set to a predetermined weight of the green compact (118.38 g).
Was adjusted to, and the continuous molding was performed.

(Example) A compact was obtained by continuously compacting a compact having a compact density of 6.7 g / cm ^{3 with} the compacting speed set to 10 pieces per minute as in the case of the conventional example. Were weighed 100%. Next, this data is divided into 10 pieces in the order of molding, and the average value of each data is obtained, and the result is shown in FIG.
Is shown in the graph. Since the procedure of the graph is the same as that of Fig. 3, its explanation is omitted, and some numerical values are shown in 2 minutes (scale 2
0) ... 118.42 g, 10 minutes ... 118.39 g, 16
Minutes ... 118.43 g, 24 minutes ... 118.37 g, 31 minutes ... 118.37 g. As is apparent from this graph, there is no increase in weight in the initial stage in the conventional method, and in the conventional example, the scales 10 to 110 showing the temperature increase.
The average value for 10 minutes and the average value for 20 minutes of 120 to 310 are 118.38 g, and a stable weight is maintained from the beginning.

[0020]

[Effects of the Invention] Conventionally, even if mass-production molding is started, for a while, the weight and other characteristics of the green compact obtained deviate from the set values and molding defects tend to occur, which requires a lot of time and effort for monitoring and adjustment. Although expensive, this invention obviates the need for such monitoring and adjustment work. Therefore, the effects obtained by implementing the present invention are to prevent molding defects, improve the efficiency of the molding process,
There are extremely large items such as stable product quality and reduced production costs.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a general method of filling raw material powder into a powder molding die.

FIG. 2 is a diagram illustrating the configuration of a die set including a powder molding die.

FIG. 3 is a graph showing changes in the weight of the green compact during continuous molding.

FIG. 4 is a graph showing how the temperature of a green compact changes during continuous molding.

FIG. 5 is a graph showing a state where the weight of the green compact is stable in the example of the present invention.

[Explanation of symbols]

10 ... Die, 20 ... Lower punch, 30 ... Upper punch,
40 ... Core rod, 11 ... Die plate, 70 ... Feeder.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-9-272901 (JP, A) JP-A-8-143904 (JP, A) JP-A-9-253896 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) B22F 3/02

Claims (2)

(57) [Claims] 1. A powder molding method in which a raw material powder to which a powder lubricant is added is filled in a die cavity and compression-molded between upper and lower punches, and the resulting green compact is extruded from a die by a lower punch. Is heated to preheat to the equilibrium temperature that the mold reaches during operation, and in that state, the filling amount of the raw material powder is adjusted to a predetermined value and continuous molding is started. A method for stabilizing the weight of a green compact in a powder molding process for a sintered part, comprising: 2. A mold for forming a die cavity,
10 to 3 from the temperature of the mold before the start of continuous molding
The method for stabilizing the weight of a green compact in the powder molding step for a sintered component according to claim 1, wherein the method is preheating to a temperature higher by 0 ° C.